Dears,

As we deal with open discharge of catagory-D fluid like steam to atmosphire through silencer from a vent or a safety valve outlet, is there any need for axial bellow or slip joint with B31.1 Appendix blowback criteria fulfilled condition is sufficient ?

Is it commonplace to anchor the silencer inlet without any bellow/ slip joint for axial expansion ?

regards,

sam

Sam,

Your questions are very general to answer.

You can release steam to atmosphere straight trough the discharge elbow of safety valve if you do not have any restriction to do that. However, most of the cases you need to extend the discharge pipe at least around 2-2.5 m above any platform not to expose people to steam.

If you can extend the discharge pipe with suitable supporting system there is no need to use a slip joint. However, most of the cases the supporting system may introduce additional loads to the safety valve under the thermal loads. This is not wanted and therefore there is a need to split the discharge pipe by a slip joint (or an expansion chamber) to control the loads on the safety valve and make discharge pipe supports adequately with different expansion behavior. In this case you need to check the discharge pipe (diameter) beyond the split joint for blowback. In the blowback calculation you need to consider the silencer loss (sometimes it may be governing the entire loss and changing the safety valve requirements for back pressure as well). You may ask silencer vendor for the acceptable loss limitation to design for. It is the cost driven and availability decision for the safety valve and the silencer.

Sometimes you may only use a silencer that it may be part of the piping and not require additional support on structure depending on the noise restrictions.
Sometimes the noise restrictions might be requiring large size silencer that you may not be able to support them through the pipe any more. It may be very heavy and very large. So, you need to support the silencer on the structure and may/may not need to use another expansion chamber or bellows to eliminate unwanted forces under differential axial and lateral expansions at the connection between pipe and silencer. This requires close attention by the engineer who designs the system.

I hope this answers your questions.

Ibrahim Demir